The present invention relates to lithium ion batteries. In particular, it is related to lithium ion batteries containing a liquid electrolyte and at least one ionically conductive and electrochemically active polyimide-based electrode.
Typical rechargeable lithium cells use lithium metal electrodes as an ion source in conjunction with positive electrodes. These positive electrodes comprise compounds capable of intercalating the lithium ions within their structure during discharge of the cell. These cells rely on separator structures or membranes that physically contain a measure of fluid electrolyte, usually in the form of a solution of a lithium compound. The separator structure also provides a means for preventing destructive contact between the electrodes of the cell. Sheets or membranes ranging from glass fiber filter paper or cloth to microporous polyolefin film or nonwoven fabric have been saturated with solutions of a lithium compound, such as LiClO4, LiPF6, or LiBF4, in an organic solvent such as propylene carbonate, diethoxyethane, or dimethyl carbonate, to form an electrolyte/separator element. A fluid electrolyte bridge is thus established between the electrodes and effectively provides the necessary Li++ ion mobility at conductivities in the range of about 10xe2x88x923 S/cm.
Gozdz et al. (U.S. Pat. No. 5,460,904) point out that these separator elements unfortunately comprise sufficiently large solution-containing voids establishing continuous avenues between the electrodes. In turn, lithium dendrite formation is enabled during charging cycles and eventually internal cell short-circuiting occurs. To combat this problem, lithium-ion cells have been made where both electrodes comprise intercalation materials, such as lithiated metal oxides, graphites, and carbons. This eliminates the lithium metal which promotes the deleterious dendrite growth. However, these cells do not attain the capacity provided by lithium metal electrodes.
Gozdz et al. proposed an electrolytic cell electrode and separator elements that employ a combination of poly(vinylidene fluoride)copolymer matrix and a compatible organic solvent plasticizer which maintains a homogeneous composition in the form of a flexible, self-supporting film. The copolymer comprises about 75 to 92% by weight vinylidene fluoride (VdF) and 8 to 25% hexafluoropropylene (HFP). The HFP limits the crystallinity of the final copolymer to a degree such that it ensures good film strength while enabling the retention of about 40 to 60% of preferred solvents for lithium electrolyte salts. Within this range of solvent content, the 5 to 7.5% salt ultimately comprising a hybrid electrolyte membrane yields an effective room temperature ionic conductivity of about 10xe2x88x924 to 10xe2x88x923 S/cm, yet the membrane exhibits no evidence of solvent exudation which might lead to cell leakage or loss of conductivity.
Each electrode is typically prepared from a collector foil in the form of an open mesh, upon which is laid either a positive or a negative electrode membrane. This membrane comprises an intercalatable material dispersed in a polymeric binder matrix such as poly(vinylidene fluoride) or poly(tetrafluoroethylene). The binder matrix provides no electrochemical benefit to the electrode and functions strictly to hold the intercalatable materials to the collector foil while the electrodes are exposed to the liquid electrolyte. Typically, these binders are fluorinated polymers.
The use of fluorinated polymers proves to be destructive to the cell because lithium has a tendency to react with the fluorine in the polymer to form lithium fluoride. This reduction leads to degradation of performance since the lithium ions are removed from the charge/discharge reaction. In addition, the fluorinated polymers may decompose to generate hydrogen fluoride which reacts vigorously and exothermically with the lithium salt to degrade or halt the operation of the battery. Although the currently used binders have good cohesive properties for holding or consolidating particles, they are poor adherents for binding particles to the metal current collectors. Some of these binders also contain moisture which reacts with the lithium salts and degrades performance. Lastly, some of the binders cannot withstand exposure to high temperatures. Therefore, the useful temperature range for the battery is limited.
Fujimoto et al. (U.S. Pat. No. 5,468,571) addressed the temperature problem by providing a secondary battery wherein the negative electrode is prepared with a polyimide binder. However, the polyimides used by Fujimoto et al. are condensation type polyimides which require a dehydration condensation reaction. If the dehydration condensation reaction has not been driven to completion, water may be released when the battery temperature becomes abnormally high. This residual water reacts vigorously with lithium. Although polyimides exhibit good binding and adhesion properties, Fujimoto et al. observed that use of polyimides in excess of 2 parts by weight caused a decrease in capacity.
Gan et al. (xe2x80x9cThe Effect of Binder Type on Li-Ion Electrode Performancexe2x80x9d, 15th International Seminar and Exhibit on Primary and Secondary Batteries, Mar. 3, 1998, pp. 1-12.) studied the use of polyimides as binders for both anodes and cathodes. They observed that graphite (anodes) electrodes with polyimide binder exhibited high irreversible capacities and the higher the polyimide concentration, the larger the irreversible capacity. However, they also noted that although graphite anodes containing polyimide binder showed reasonably good adhesion to the foil substrate, they were much more brittle and prone to cracking than the PVDF-type electrodes. For the cathode, it was found that test cells having a polyimide (xe2x89xa73.6%) binder had practically no charge capacities and could not be cycled. In addition, when the binder content was reduced, the test cells continued to not cycle well. It was concluded that cathodes using polyimide binders were more brittle than the other cathodes and suffered from cracking.
Gustafson et al. (U.S. Pat. No. 5,888,672) disclose a battery where the anode, the cathode, and the electrolyte each comprise a soluble, amorphous, thermoplastic polyimide. Since the polyimides are pre-imidized prior to the fabrication of the battery, there is no need to further cure them at high temperatures, thus reducing the risk of damaging the battery. Nor is there a chance of incidental condensation as the battery temperature rises. In addition, since no further polymerization will occur, there are no byproducts of the condensation reaction (water) to interact with the lithium salts. The battery of Gustafson et al. is a dry cell.
In fabricating the battery, a minimal amount of pressure or an adhesive is applied to the laminate to allow for intimate lateral contact to be made between the layers. Ultimately, a uniform assembly is formed that is self-bonded and exhibits adhesion between the layers. Since the polyimides used are amorphous, there is an unobstructed pathway for ionic mobility. However, the battery of Gustafson et al. requires bonding or application of an adhesive (prepared from the electrolyte solution) between the layers to promote an unobstructed pathway for ionic mobility. If there are any gaps or defects between the layers, the ionic pathway is upset and the battery function is impaired.
An object of the present invention is to provide a polyimide-based battery wherein the ionic conductivity is insured through the use of a solid electrolyte polyimide binding material.
Another object of the present invention is to provide a polyimide-based battery having at least one ionically conductive and electrochemically active electrode.
Another object of the present invention is to provide a polyimide-based battery that has excellent high temperature stability.
Another object of the present invention is to provide a polyimide-based battery that has a low (less than 1%) initial fade rate over 50 cycles.
Another object of the present invention is to provide a polyimide-based battery where the anode and/or the cathode are not brittle.
Another object of the present invention is to provide a polyimide-based battery having good cohesive properties within the electrolyte film layers.
Another object of the present invention is to provide a polyimide-based battery having good adhesion of the electrode films to the metal current collectors of the cell.
Another object of the present invention is to provide a polyimide-based battery which has reduced HF formation in comparison to prior art batteries.
Another object of the present invention is to provide a polyimide-based battery which is less sensitive to overcharging and discharging than prior art batteries.
The foregoing and other objects were achieved by the present invention which is a battery having at least one anode; at least one ionically conductive and electrochemically active polyimide-based cathode; at least one separator film disposed between each anode and each cathode; and a liquid electrolyte distributed throughout each anode, each cathode, and each separator film. The cathode comprises a cathode current collector; a metal oxide; an electronic conductive filler; and an ionically conductive and electrochemically active cathode solid electrolyte polyimide binder. For the purpose of this specification and the appended claims, a solid electrolyte is defined as an electrolyte medium that does not contain solvent but provides for the transfer of lithium ions. In other words, the electrolyte medium is dry or contains no solvent. The ionically conductive and electrochemically active cathode solid electrolyte polyimide binder comprises a lithium salt and a pre-imidized soluble, amorphous, thermoplastic polyimide powder. Both the lithium salt and the pre-imidized soluble, amorphous, thermoplastic polyimide powder are soluble in any polar solvent. Preferably, the polar solvent is selected from the group consisting of: N-methylpyrolidinone (NMP), N,N-dimethylacetamide (DMAc), tetrahydrofuran (THF), cyclohexanone, 1,4-dioxane, chloroform, acetophenone, ethylene chloride, gamma-butyrolactone, m-cresol, methylene chloride, methyl ethyl ketone, methoxypropanol, and dimethylformamide (DMF). The ionically conductive and electrochemically active cathode may be combined with any anode, such as a composite anode or a lithium metal anode; a separator film; and any liquid electrolyte known to those skilled in the art to form the battery of the present invention.
In another embodiment of the invention, the anode is ionically conductive and electrochemically active. The anode comprises an anode current collector; an electronic conductive filler; an intercalation material; and an ionically conductive and electrochemically active anode solid electrolyte polyimide binder. The ionically conductive and electrochemically active anode solid electrolyte polyimide binder comprises a lithium salt and a pre-imidized soluble, amorphous, thermoplastic polyimide powder. As with the cathode previously described, both the lithium salt and the pre-imidized soluble, amorphous, thermoplastic polyimide powder are soluble in any polar solvent known to those of ordinary skill in the art and, in particular, the solvent is either N-methylpyrolidinone (NMP), N,N-dimethylacetamide (DMAc), tetrahydrofuran (THF), cyclohexanone, 1,4-dioxane, chloroform, acetophenone, ethylene chloride, gamma-butyrolactone, m-cresol, methylene chloride, methyl ethyl ketone, methoxypropanol, and dimethylformamide (DMF) or various combinations thereof. The ionically conductive and electrochemically active anode may be combined with any cathode, separator film, and liquid electrolyte known to those skilled in the art or it may be used with the ionically conductive and electrochemically active cathode previously described to form a liquid electrolyte lithium ion battery.
The polyimide-based lithium ion batteries of the present invention are more energy efficient than prior art lithium ion batteries. Since at least one of the electrodes is ionically conductive and electrochemically active, the overall battery performance is enhanced. The batteries of the present invention have a longer cycle life and run time than those of the prior art. In particular, they exhibited an unexpectedly lower initial fade rate (0.06% fade rate over 50 cycles as compared to a 0.23% fade rate over 50 cycles) than the prior art batteries. In addition, the batteries of the present invention are less sensitive to over-charging and over-discharging of battery systems, thus safeguarding against thermal runaway. This minimizes the need for additional electronic safety systems, thereby reducing the overall cost for producing the battery. These types of batteries are particularly useful for portable electronic devices such as laptop computers, camcorders, super-capacitors, and cellular telephones as well as electronic chips.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be obtained by means of instrumentalities in combinations particularly pointed out in the appended claims.